This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-354476, filed Dec. 14, 1999, the entire contents of which are incorporated herein by reference.
The present invention relates to a bias circuit of a transistor, and more particularly to a base bias circuit of a bipolar transistor for the high frequency power amplification.
A structure of a conventional base bias circuit of a bipolar transistor will be explained by using FIG. 10. The conventional base bias circuit 5 shown in FIG. 10 comprises a diode Di for the temperature compensation connected in series, and a resistor R for the temperature compensation. A cathode of the diode Di for the temperature compensation is grounded, and an anode thereof is connected to one terminal of the resistor R for the temperature compensation, and a reference voltage Vref is applied to the other terminal of the resistor R for the temperature compensation.
An output voltage Vout of the base bias circuit 5 is output from the connection point of the diode Di for the temperature compensation and a resistor R for the temperature compensation to supply a base voltage Vb and a base current Ib of a bipolar transistor Qh for the high frequency power amplification shown on the right side of FIG. 10.
An emitter of the bipolar transistor Qh for the high frequency power amplification is grounded and a collector voltage Vc is applied to a collector of the bipolar transistor Qh. A high frequency signal RFin is input to the base via a DC-cut capacitor, so that a base current Ib of the bipolar transistor Qh is modulated with this high frequency signal RFin.
The base current Ib which is high frequency modulated is amplified with the bipolar transistor Qh for the high frequency power amplification, and the amplified base current Ib modulates a collector current Ic at a large amplitude. A high frequency signal is output from the collector current Ic which is high frequency modulated via the output circuit. A structure of the output circuit for outputting the high frequency signal is not directly associated with the present invention, and the structure is omitted in FIG. 10.
Next, an operation of the bias circuit 5 shown in FIG. 10 will be explained. Here, there will be described a thermal runaway of the bipolar transistor for the high frequency power amplification which has conventionally become a problem besides, countermeasures thereof will be described as well. In order to obtain a high power high frequency signal by using the bipolar transistor, the collector current must be increased. However, when the collector current is increased, a large collector loss is generated. When the collector loss becomes large, a temperature of the bipolar transistor rises. When the temperature rises, the collector current increases. The increase in the collector current further increases the temperature of the bipolar transistor with the result that the collector current unusually increases and the bipolar transistor generates a thermal runaway.
The conventional bias circuit 5 shown in FIG. 10 is intended to suppress an increase in the collector current Ic of the bipolar transistor Qh for the high frequency power amplification resulting from the temperature rise by using the resistor R for the temperature compensation and the diode Di for the temperature compensation. A rise of current in the forward direction in the current voltage characteristic of the diode Di for the temperature compensation is shifted toward the side of the low voltage together with the temperature rise with the result that an equivalent resistance Rdi of the diode Di for the temperature compensation lowers together with the rise of the temperature.
On the other hand, the output voltage Vout of the bias circuit 5 can be obtained by dividing the reference voltage Vref with the equivalent resistance Rdi of the diode Di for the temperature compensation and the diode Di for the temperature compensation so that the output voltage Vout is lowered together with the temperature rise. With the lowering of the output voltage Vout, the base voltage Vb of the bipolar transistor Qh is lowered and the base current Ib decreases with the result that an increase in the collector current Ic resulting from the temperature rise is suppressed.
For preventing the thermal runaway, a method is conventionally used which comprises the steps of inserting the diode Di for the temperature compensation into the bias circuit of the bipolar transistor, and suppressing the increase in the collector current Ic resulting from the temperature rise. However, while this method is effective for the prevention of the thermal runaway, there arises a serious problem in that an increase in the output power Pout of a high frequency signal is suppressed with respect to the input power Pin of the high frequency signal RFin because a resistor R for the temperature compensation is inserted. It is the most important as the performance of the bipolar transistor Qh for the high frequency power amplification that the output power Pout of a high frequency signal is increased with respect to the input power Pin of the high frequency signal RFin.
When described in detail, the high frequency signal RFin is input and the bipolar transistor Qh for the high frequency power amplification is operated in a non-linear region of the high frequency input powerxe2x80x94output power characteristic, the high frequency current which flows through the bipolar transistor Qh is subjected to a kind of rectifying action with the result that the base current Ib is increased as a result. When the bias circuit 5 shown in FIG. 10 is not connected to the bipolar transistor Qh for the high frequency power amplification, the increase in the base current Ib increases the collector current Ic so that the output saturation is alleviated which results from non-linear characteristic of the high frequency input powerxe2x80x94output power characteristic of the bipolar transistor Qh for the high frequency power amplification and the high frequency output power characteristic so that an effect that the output power Pout of the high frequency signal is amplified can be obtained, which effect is extremely desirable for the high frequency power amplifier.
However, when the bias circuit 5 shown in FIG. 10 is connected to the bipolar transistor Qh for the high frequency power amplification, an increase in the base current Ib generated in the operation in a non-linear region of the high frequency input powerxe2x80x94output power characteristic of the bipolar transistor Qh for the high frequency power amplification increases a current Ir which flows through a resistor R for the temperature compensation, so the output voltage Vout of the bias circuit 5 and the base voltage Vb of the bipolar transistor Qh for the high frequency power amplification are lowered. As a consequence, there arises a problem in that an increase in the output power Pout of the high frequency signal resulting from the operation in the non-linear region of the high frequency input powerxe2x80x94output power characteristic of the bipolar transistor Qh for the high frequency power amplification is eliminated, which is a very serious problem that cannot be ignored as the high frequency power amplifier.
As described above, the conventional bias circuit of the bipolar transistor for the high frequency power amplification is effective for the prevention of the thermal runaway resulting from the temperature rise. On the other hand, however, there is a serious problem which cannot be ignored as the high frequency power amplifier such that an increase in the output power of the high frequency signal is eliminated which is based on a kind of rectifying action shown by the bipolar transistor for the high frequency power amplification in the operation in the non-linear region of the high frequency input powerxe2x80x94output power characteristic.
The present invention has been made in order to solve the problem, and an object of the invention is to provide a bias circuit of a bipolar transistor for the high frequency power amplification which does not eliminate an increase in an output power of a high frequency signal in a non-linear operation region of a high frequency input powerxe2x80x94the high frequency output power characteristic while preventing, the thermal runaway resulting from the temperature rise.
The bias circuit of a transistor according to the present invention is characterized by comprising a circuit which is capable of conducting the temperature compensation without eliminating an increase in the base current at the time of inputting a high frequency signal when the bipolar transistor for the high frequency power amplification is operated in a non-linear region having a high frequency input powerxe2x80x94output power characteristic.
According to a first aspect of the present invention, there is provided a bias circuit of a transistor comprising a first transistor, a second transistor, a first resistor and a second transistor, the first transistor being a bipolar transistor, the second transistor being a field effect transistor; wherein an emitter of the bipolar transistor is grounded; a collector and a base of the bipolar transistor are connected to each other, a connection point between this collector and the base are connected to one of the terminals of the first resistor; a control voltage is applied to the other terminal of the first resistor, and the connection point between the collector and the base of the bipolar transistor is connected to the gate of the field effect transistor; a source of the field effect transistor is connected to one terminal of the second resistor; the other terminal of the second resistor is grounded; a reference voltage is applied to the drain of the field effect transistor; and a bias voltage is output from the connection point between the source of the field effect transistor and one terminal of the second resistor.
In the bias circuit of a transistor according to the first aspect of the present invention, the field effect transistor may be a depletion type field effect transistor.
In the bias circuit of a transistor according to the first aspect of the present invention, the bipolar transistor may be an NPN transistor, and the field effect transistor may be an N-channel transistor. The field effect transistor may be a depletion type field effect transistor.
According to a second aspect of the present invention, there is provided a bias circuit of a transistor comprising a diode, a transistor, and a first and a second resistor; wherein the transistor is a field effect transistor; a cathode of the diode is grounded; an anode of the diode is connected to one terminal of the first resistor; the control voltage is applied to the other terminal of the first resistor; a connection point between the anode of the diode and one terminal of the first resistor is connected to the gate of the field effect transistor; the collector and the base of the bipolar transistor are connected to each other, and a connection point between the collector and the base is connected to one terminal of the first resistor; a control voltage is applied to the other terminal of the first resistor; a connection point between the anode of the diode and one terminal of the first resistor is connected to the gate of the field effect transistor; a source of the field effect transistor is connected to one terminal of the second resistor; the other terminal of the second resistor is grounded; a reference voltage is applied to a drain of the field effect transistor; and a bias voltage is output from the connection point between the source of the field effect transistor and one terminal of the second resistor.
In the bias circuit of a transistor according to the second aspect of the present invention, the field effect transistor may be a depletion type field effect transistor.
In the bias circuit a transistor according to the second aspect of the present invention, the field effect transistor may be an N channel transistor. The field effect transistor may be a depletion type field effect transistor.
According to a third aspect of the present invention, there is provided a bias circuit of a transistor comprising a first transistor and a second transistor, a first resistor and a second resistor, the first transistor being a bipolar transistor, the second transistor being a bipolar transistor: wherein an emitter of the bipolar transistor constituting a first transistor is grounded; a collector and a base of a bipolar transistor constituting a first transistor are connected to each other, and a connection point between this collector and the base is connected to one terminal of the first resistor; a control voltage is applied to the other terminal of the first resistor; a connection point between the collector and the base of the bipolar transistor constituting the first transistor is connected to a base of the bipolar transistor constituting the second transistor; an emitter of the bipolar transistor constituting the second transistor is connected to one terminal of the second resistor; the other terminal of the second resistor is grounded; a reference voltage is applied to the collector of the bipolar transistor constituting the second transistor; and a bias voltage is output from the connection point between the emitter of the bipolar transistor constituting the second transistor and one terminal of the second resistor.
In the bias circuit of a transistor according to the third aspect of the present invention, the bipolar transistor constituting the first transistor may be an NPN transistor, and the bipolar transistor constituting the second transistor may be the NPN transistor.
According to a fourth aspect of the present invention, there is provided a bias circuit of a transistor comprising a diode, a bipolar transistor, a first resistor and a second resistor: wherein a cathode of the diode is grounded; an anode of the diode is connected to one terminal of the first resistor; a control voltage is applied to the other terminal of the first resistor; a connection point between the anode of the diode and the first resistor is connected to a base of the bipolar transistor; an emitter of the bipolar transistor is connected to one terminal of the second resistor; the other terminal of the second resistor is grounded; a reference voltage is applied to the collector of the bipolar transistor; and a bias voltage is output from the connection point between the emitter of the bipolar transistor and one terminal of the second resistor.
In the bias circuit of a transistor according to the fourth aspect of the present invention, the bipolar transistor may be an NPN transistor.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.